The manufacturing environment in which silicon semiconductor wafers are formed and further processed is inherently dirty. Attempts are made to keep the surrounding environment as clean as possible, but some particles always escape these attempts. Particles that may end up on the surface of a silicon semiconductor wafer include such things as silicon chips, silicon dioxide particles, organic particles (such as those shed by human beings including skin particles, dandruff, and etc.), and particles containing calcium, magnesium, chlorine, sulphur and etc. These particles have a tendency to interfere with the functioning of the semiconductor circuits being manufactured.
For example, if it is desired to form a thin film over a silicon wafer, and a particle of unwanted material is on the surface, the coating will be incomplete. If the purpose of the coating is to isolate one layer from another, the particle may serve as a hole or as a conductor to connect the two surfaces in an unwanted manner. Also if particles are present on the surface during an etching process, the surface under the particle will not be etched and, therefore, will create a short in the circuit. If the particle is in the size range of 10-20% of one of the feature sizes of the circuit, the particle is likely to be a problem. It is, therefore, necessary to remove unwanted particles by some additional method.
This becomes a problem as the size of the particles gets smaller and smaller. It is nearly impossible, using current technology, to remove a particle that is 0.1 of a micron or smaller in size. This is due to the fact that the adhesive forces on the particle are greater than the available area for which a removal force may be applied to the particle. Prior attempts to overcome such adhesive forces include different methods of applying a physical force to the particle, all of which have proven to be ineffective against very small particles.
One previously developed method of particle removal is to place the surface to be cleaned in a chemical bath and then use an ultrasonic or a megasonic sound agitating system. In this system, the ultrasonic or megasonic sound is induced into the liquid which shakes the chemical bath and attempts to remove any particles. This has been somewhat effective against relatively large particles, but ineffective against small particles.
Another cleaning method which has been previously used is to blast the surface to be cleaned with a fluid in order to blow or wash the particles from the surface. One such method utilizing water is disclosed in U.S. Pat. No. 4,027,686, June 7, 1977, to Shortes, et al. and assigned to Texas Instruments, Inc. Other fluids, including air, have been used with varying degrees of success. These methods have proven to be effective against large particles, but relatively ineffective against particles in the 0.1 micron or smaller range.
Another previously developed method involving application of force to the particle is to exploit the mass of the particle by the use of gravity or centrifugal force. These methods require the particle be of sufficient size to be pulled or thrown from the surface. When the particles are very small or have little mass the forces may not be high enough to remove them. Therefore, these methods are generally ineffective against submicron sized particles.
Still another line of previous attempts to remove particles is to convert them into nonparticles. This requires a knowledge of the chemistry of the particles, or a universal solvent (one that would dissolve all particles). This method uses a chemical specifically designed to attack and dissolve the particles but not the substrate they are on. It is necessary to know what the particles to be removed are and, therefore, the appropriate chemical. Since it is unlikely that the composition of all the particles will be known, it is difficult to remove all the particles with a chemical.
Another prior method to remove particles involves particle and surface interaction. This is an attempt to make the particle and the surface repel each other through the use of electricity. The problem encountered with this method is that a high electrical charge is required and, thus, it has been found to be undesirable.
One of the most common methods of particle removal involves using a brush scrubber. In this method, a brush is used to wipe the particles from the surface. It is necessary to compromise in the stiffness of the brush bristles, in that they must be stiff enough to remove the particles yet soft enough so as to not damage the surface. This method is effective against relatively large particles only.
A final method of removing particles has been an attempt to adhere the particles to a removable film. This method can be likened to the use of masking tape to remove lint from a jacket. A polymer gel or liquid is applied to the surface in an attempt to soak up all the particles. After the polymer has dried, and hopefully drawn the particles into it, the polymer is peeled off to remove the particles. This method, as with the other methods listed above, works only with relatively large particles.
Another handicap of these previously developed physical removal processes is the fact that they can damage the surface being cleaned as well as introduce other particles while trying to remove them.
Thus, a need has arisen for a method of removing very small particles from a semiconductor surface.